Measurement of radio transparency of polycrystalline CVD-diamond in millimeter-wave range by free-space method

Radio transparency of polycrystalline CVD-diamond disks with diameter up to 75 mm in millimeter-wave range was measured by free-space method. The structure of the disks was characterized by Raman spectroscopy and scanning electron microscopy. Dielectric loss tangent $\tan\delta$ of the samples in the frequency range of 50 – 67 GHz was found to be in the range of 7.5 $\cdot$ 10$^{-3}$ – 8 $\cdot$ 10$^{-2}$, increasing with frequency. The transmission loss due to the radiation absorption is about 1%.